1. Field of the Invention
The present invention relates to an exhaust gas recirculation system for an engine with a turbocharger.
Recirculation of exhaust gas into the intake gas of an internal combustion engine is widely recognized as a significant method for reducing the production of nitrous oxides (NO.sub.x) during the combustion process. The recirculated exhaust gas partially quenches the combustion process and lowers the peak temperature produced during combustion. Since NO.sub.x formation is related to peak temperature, recirculation of exhaust gas reduces the amount of NO.sub.x formed.
In order to recirculate exhaust gas into the intake manifold, the exhaust gas must be at a pressure that is greater than the pressure of the intake gas. However, in a turbocharged engine, the intake gas is typically at a pressure higher than that of the exhaust gas. This is because the compressor of the turbocharger purposefully increases the pressure of the intake gas while producing only a signal increase in the pressure of the exhaust gas. Therefore, the pressure differential from exhaust gas, to intake gas for a turbocharged engine, is often in the wrong direction to have flow from the exhaust system to the intake system. This is especially true at peak torque operation, where the inlet system is significantly boosted in pressure, and the combustion temperatures are relatively high.
Various systems have been proposed to provide recirculated exhaust gas for an engine with a turbocharger. In one design, a butterfly-type valve is placed at the exhaust of the turbine so as to backpressure the exhaust system. However, this backpressuring of the turbine results in a thrust load on the turbocharger rotor system which increases wear of the turbocharger bearings. Furthermore, exhaust gas under pressure may leak past the seals and bearings of the turbocharger and flow with the returned lubricating oil into the crankcase of the engine, undesirably increasing engine crankcase pressure.
In yet another design, a combination backpressure/EGR valve is provided at the turbine inlet. In this valve, the actuation of the backpressure valve is mechanically locked to the actuation of the EGR valve. This valve does not include flexibility to change the backpressuring of the exhaust system independently of the recirculation of exhaust gas. This lack of flexibility means that the amount of exhaust gas recirculated may be optimized for a single or narrow range of conditions, and would be non-optimum for most conditions. Also, such a combination valve does not permit backpressuring of the exhaust system during cold start up of the engine without also permitting a high degree of recirculated exhaust gas flow. Such a combination valve would be less than optimum for reduction of white smoke from a diesel engine during cold starting and warm-up.
What is needed is an exhaust gas recirculation system that provides improvements to current systems. The present invention provides a novel and unobvious EGR system that provides those improvements.